Provide low level segments information that a Lucene index (shard level)
is built with. Allows to be used to provide more information on the
state of a shard and an index, possibly optimization information, data
"wasted" on deletes, and so on.

Endpoints include segments for a specific index, several indices, or
all:

The key of the JSON document is the name of the segment. This name
is used to generate file names: all files starting with this
segment name in the directory of the shard belong to this segment.

generation

A generation number that is basically incremented when needing to
write a new segment. The segment name is derived from this
generation number.

num_docs

The number of non-deleted documents that are stored in this segment.

deleted_docs

The number of deleted documents that are stored in this segment.
It is perfectly fine if this number is greater than 0, space is
going to be reclaimed when this segment gets merged.

size_in_bytes

The amount of disk space that this segment uses, in bytes.

memory_in_bytes

Segments need to store some data into memory in order to be
searchable efficiently. This number returns the number of bytes
that are used for that purpose. A value of -1 indicates that
Elasticsearch was not able to compute this number.

committed

Whether the segment has been sync’ed on disk. Segments that are
committed would survive a hard reboot. No need to worry in case
of false, the data from uncommitted segments is also stored in
the transaction log so that Elasticsearch is able to replay
changes on the next start.

search

Whether the segment is searchable. A value of false would most
likely mean that the segment has been written to disk but no
refresh occurred since then to make it searchable.

version

The version of Lucene that has been used to write this segment.

compound

Whether the segment is stored in a compound file. When true, this
means that Lucene merged all files from the segment in a single
one in order to save file descriptors.